1. Field of the Invention
The present invention relates to a reflection type liquid crystal device suitable for use as a light valve or an optical modulation device for a projection display system and to a manufacturing method therefor, and to a projection display system employing such a liquid crystal device.
2. Description of the Related Art
Recently, a type of projection display that uses an active matrix type reflective liquid crystal device as a light valve or as an optical modulation device has been developed for use in a high image quality display. Such a projection display is disclosed, for example, in Japanese Unexamined Patent Publication (Patent Kokai) Nos. 08-248425, 07-209621 and 08-328034.
In a projection display using a reflective liquid crystal device, light emitted by a light source is separated into the three primary colors: red, blue and green. The light elements of the respective colors are transmitted to corresponding reflective liquid crystal devices where they are optically modulated. The colored light elements reflected by the liquid crystal devices are recombined and the resultant light is magnified by an optical lens system and is projected onto a screen to thereby display a color image.
An active matrix type reflective liquid crystal device has an array substrate and an opposing substrate disposed opposite the array substrate at a predetermined space. The array substrate includes switching elements which are field-effect transistors (FETs), provided in a matrix correspondingly to pixels, pixel electrodes connected to the switching elements and arranged as a matrix, and storage capacitors connected to the pixel electrodes for holding the charge on the electrodes. A transparent electrode is provided on the opposing substrate. A liquid crystal layer is inserted into the space between the array substrate and the opposing substrate.
Light is incident on the side of the opposing substrate, and the liquid crystal selectively changes the polarization state of the incident light in response to a voltage applied to the pixel electrodes. The incident light is reflected by the pixel electrodes that also serve as light reflectors, the reflected light passing through the transparent electrode and emerging from the liquid crystal device.
FIGS. 1 and 2 show a conventional active matrix type reflective liquid crystal device that is similar to that shown in FIGS. 1 and 2 of the above-mentioned Japanese Unexamined Patent Publication (Patent Kokai) No. 08-248425.
The array substrate includes a silicon substrate 1, on which field-effect transistors (FETs), are provided in a plurality of regions defined by field oxide isolation regions 12. The FETs, which are provided correspondingly to pixels, each include a gate insulating film 2 made, for example, of silicon dioxide, a gate electrode 4 made, for example, of polysilicon, a drain region 6, a source region 8, and a channel region 10 extending between the drain 6 and the source 8.
A storage capacitor 16 for holding a charge is formed on silicon dioxide film 14. The storage capacitor 16 includes two polysilicon layers that serve as capacitor electrodes and a dielectric layer made of silicon dioxide film that is sandwiched between the polysilicon layers. A silicon dioxide film 18 is formed to cover the silicon dioxide film 14 and the capacitor 16. A drain electrode 20 and a source electrode 22 made, for example, of aluminum are formed in openings in the silicon dioxide films 14 and 18. The drain electrode 20 is connected to a data line 21 that extends perpendicular to the drawing in FIG. 1, and the gate electrode 4 is connected to a gate line 5 (see FIG. 2) that is orthogonal to the data line 21.
The source electrode 22 has an extension 23 which extends on the silicon dioxide film 18 to overlap the capacitor 16. The upper capacitor electrode is connected to the source electrode extension 23 by a via 19, made, for example, of tungsten, that passes through the silicon dioxide film 18.
A silicon dioxide film 24 is deposited on the drain electrode 20 and the source electrode 22, and a light absorption layer 26 is formed thereon. The light absorption layer 26 is a composite layer including, for example, a titanium underlayer, an intermediate aluminum layer and a titanium nitride upper layer, and prevents undesirable light reflection and light transmission to the FETs.
A through hole is formed in the light absorption layer 26 to permit the passage of a via 30. A silicon nitride film 28 that is substantially about 4000 .ANG. to about 5000 .ANG. thick is formed on the light absorption layer 26. Pixel electrodes 32 made of aluminum and having a thickness of 1500 .ANG. are formed on the film 28 at a space or gap of approximately 1 .mu.m. The pixel electrodes 32 are also used as light reflectors (reflection mirrors), and the array of the pixel electrodes 32 forms a light reflective plane.
The via 30, made of tungsten, is formed so that it passes through the silicon dioxide film 24, the light absorption layer 26 and the silicon nitride film 28. The FET source electrode 22 and the pixel electrode 32 are connected to each other by the via 30. A liquid crystal molecule alignment film 33 is formed on the array of the pixel electrodes 32.
As shown in FIG. 2, the pixel electrodes 32 are arranged as a matrix to correspond to the individual pixels. Spacers 34, which are shaped like pillars about 2 to 3 .mu.m tall and are made, for example, of silicon dioxide, are provided at selected positions between the pixel electrodes 32.
Disposed on the pillar-shaped spacers 34 is an opposing substrate comprising a glass substrate 40 on which is located a transparent opposing electrode or common electrode 38 coated with a liquid crystal molecule alignment film 37. The transparent electrode is made, for example, of ITO (indium tin oxide). A liquid crystal layer 36 is inserted between the array substrate and the opposing substrate.
The aluminum pixel electrodes 32 are formed by depositing aluminum on the entire surface of the silicon dioxide film 28, and by etching the aluminum layer using a photolithographic process. Then, a liquid crystal molecule alignment film, such as polyimide film, is formed to cover the array of the pixel electrodes 32, and rubbing (e.g., polishing) of the alignment film is performed.
FIG. 3 is an enlarged cross-sectional view of portion A enclosed by the circle shown in FIG. 2. As shown in FIG. 3, a height difference (e.g., a groove) exists between the pixel electrodes 32. Since silicon nitride film is not fully resistant to RIE (Reactive Ion Etching), which is used for aluminum etching, the silicon nitride film is also more or less etched during the aluminum etching.
As a result, a groove or a height difference equal to the sum of the thickness of the pixel electrode 32 and the depth of the cut in the etched silicon nitride film is formed in the area between the pixel electrodes 32.
In the above-described conventional structure, there are several problems.
First, light incident on the edges of the pixel electrodes is scattered. The scattered light does not effectively act as light that constitutes a pixel, and causes a loss in reflected light.
In addition, the regions between the pixel electrodes do not act as an effective light reflector. When adjacent pixel electrodes are driven simultaneously, an electric field similar to that applied across the liquid crystal on the pixel electrodes is also applied across the liquid crystal between the pixel electrodes, and the liquid crystal between the pixel electrodes behaves optically in a similar manner to the liquid crystal on the pixel electrodes.
That is, the liquid crystal regions on the adjacent pixel electrodes and the liquid crystal regions between the adjacent pixel electrodes behave as if they are continuous. Therefore, if the light reflector were continuous, the reflected light from the regions between the pixel electrodes could contribute to an increase in the light output. However, since the area between the pixel electrodes cannot act as an effective light reflector, the efficiency of light utilization is reduced.
Further, since the light reflective plane provided by the array of the pixel electrodes is not planar, the planarity of the alignment film deposited over the pixel electrodes is deteriorated. As a result, the alignment film is not evenly polished, which may cause poor liquid crystal alignment.
A possible method for resolving the above problems is to fill the gap regions between pixel electrodes with an insulating material, and planarize the surface by chemical-mechanical polishing (CMP) process. However, when the CMP process is used, the center portions of the relatively large pixel electrodes become recessed like a dish (e.g. "dishing"), and the reflectivity is reduced. Therefore, this is not a preferable method. Further, even if a dielectric light reflective film is formed to cover the array of the pixel electrodes, it is difficult to obtain a completely planar surface.